The oxidation of a lubricating oil often leads to the formation of oil-insoluble products which form particles that separate from the oil in a form of sludge, lacquer or varnish-like coating on engine parts.
Neutral and overbased alkaline-earth metal phenate sulfides have been known for their excellent dispersing properties and the ability to neutralize engine acids. Use has been found for these compositions primarily as additives in lubricating oils.
Overbased metal phenate sulfides contain high levels of alkali or alkaline earth metals. The measure of their alkalinity is determined by titration with acid (such as perchloric acid) and represented by the total base number (TBN). The TBN is the milligram equivalent of potassium hydroxide per gram of sample. They possess strong detergent power and an even greater capacity to neutralize acidic contaminants than the neutral metal phenate sulfides. However, overbased metal phenate sulfides which contain high levels of metal have been found to be undesirably viscous in many cases. One theory is that the high viscosity may be related to a higher solids content of overbased products which possibly contain calcium carbonate in particulate form as well as the likely presence of multivalent metal salts which may crosslink with other molecules resulting in a more viscous product. Support for this theory is in a finding that above a certain level of alkalinity undissolved solids become an increasing problem. It would be desirable to increase the alkalinity level of these additives without the high viscosity problems.
The conventional method of making overbased phenate sulfide detergents involves mixing long chain alkyl substituted phenols, a source of sulfur and an excess of metallic base. Carbonation of the reaction mass follows to increase the amount of metal base homogeneously dispersed in the final product.
In U.S. patent application Ser. No. 753,133 there is described metal phenate sulfides and a method of making metal phenate sulfides by incorporating a dispersant, preferably a metallic dispersant in the reaction mixture. The phenate sulfide reaction product exhibits improved viscometric properties and higher alkalinity.
European patent application No. 90301684.1 describes a method of making overbased phenate sulfides by including a carboxylic acid in the reaction mixture. The carboxylic acid is described as increasing the total base number (TBN) of the phenate sulfide product. Disadvantages encountered in making phenates in a manner based on the process described include the production of substantial amounts of undissolved solids prior to filtration causing slow filtration and foaming during vacuum stripping. These are serious impediments to commercial manufacture. Moreover, the products display a tendency to form a precipitate upon aging.
A faster, more efficient process for making overbased phenates would be economically advantageous Thus, a product with reduced solids levels which filters rapidly and does not produce foam while possessing a higher alkalinity at a suitable viscosity would improve processing efficiency and performance results.